The present invention relates in general to a notch filter and in particular to an improved notch filter utilizing a single port unidirectional surface acoustic wave transducer as an impedance element in a first parallel-tuned circuit which is primarily a reactive impedance at all frequencies except the center frequency at which point the parallel-tuned circuit becomes essentially resistive due to the action of the impedance element and at that point its transmission is cancelled by the transmission from a second circuit that is 180.degree. out of phase thereby providing a notch or band stop filter.
A notch filter is a device which is generally presumed to pass all frequencies except for a very narrow band which is "notched out". This type of filter is sometimes called a band elimination filter as well. In practice, the notch filter may have only a limited band for passing signals but typically this band width is several octaves wide.
Notch filter circuits are widely used in television receivers in the IF section to eliminate discrete frequency regions where unwanted carriers appear from adjacent channels. Notch filters are also used widely in cable television systems to deny access to certain signals or alternatively to remove interference in other signals.
Important characteristics of a notch filter are:
1. The amount of insertion loss in its pass region. PA0 2. The flatness of the amplitude response in the band pass region. PA0 3. The depth of the rejection notch. PA0 4. The width of the rejection notch at its ultimate rejection level. PA0 5. The width of the rejection notch measured 3 db. down from the band pass region of the notch.
Typically it is desired to have the lowest possible loss and no ripple in the pass region, as much depth at the ultimate point in the notch as possible, good width at the bottom of the notch and very narrow width at the 3 db. points on the notch.
Notch filters are generally implemented with inductors and capacitors (L's and C's) and can take one of many forms. Commonly used circuits include a shunt notch, a series notch and a bridged-T notch. A shunt notch is one in which a series resonant circuit is in parallel with the desired transmission path. This approach shunts the undesired band to ground. A series notch is one in which a parallel-tuned circuit is placed in series with the transmission line and the high impedance of the parallel-tuned circuit at resonance effectively blocks the frequency of interest from the output. For both of these circuits, the notch depth is directly related to the parasitic loss (or Q) of the inductors and capacitors used to implement the notch. The bridged-T notch circuits are an improvement on the simple series and shunt notches. Theoretically, infinite attenuation is achievable at a single frequency with a bridged-T notch if all components are chosen properly. This is achieved in spite of the finite Q in the inductors and capacitors which are used.
With higher frequencies, particularly into the ultra-high frequency range, narrower and narrower notches are generally desired when measured as a fraction of a center frequency because of the need to eliminate carriers or undesired signals which are much closer to the desired signals when measured as a fraction of the center frequency. However, inductors and capacitors are prone to temperature drift and have poor Q at UHF frequencies and the net result is that notch filters implemented using LC components tend to drift off of the desired frequency and in many cases the notch width is too broad or the filter loss is too high or both. For frequencies above approximately 150 megahertz, notch circuits are generally not utilized because of these problems.
SAW devices inherently have much higher Q's than LC filter elements and further they generally exhibit much superior temperature stability as compared to LC elements. For both of these reasons it is desirable to implement notch filters using SAW devices particularly in the UHF band.
The difficulty in implementing a notch filter with SAW devices stems from the fact that SAW devices are of themselves band pass elements in their normal configuration in which one transducer launches a wave and a second transducer receives the wave. Thus to produce a notch characteristic the devices must be used in conjunction with some other circuit which converts the band pass region of the SAW to a band reject region and vice versa.
In general the transmission characteristics of a SAW device exist only over a narrow band of frequencies which is determined by the number of electrodes and their particular placement on the transducers' surface.
In using a SAW device for a notch application, the SAW device has typically been used as one leg in some bridge circuit in which balancing of the bridge occurs at the center of the frequency of the SAW device and hence no transmission through the bridge circuit occurs.
The SAW device can either be used as a transmission element i.e. two-port element in a bridge, or as a one-port impedance element in a bridge. When used as a transmission element in a bridge, the notch filter typically has high transmission loss because the normal loss of the SAW device is very large. The exception to this case occurs when a low loss SAW device (such as a two-port resonator or low-loss transversal SAW filter) is used.
The other method uses the SAW device as an impedance element as disclosed by D. P. Akiit in "70 MHz Surface-Acoustic-Wave Resonator Notch Filter", Electronics Letters, 29 Apr. 1976, Volume 12, No. 9, pages 217 and 218. He describes a notch filter where a SAW device is used as an impedance element. Also Y. Koyamada et al in "Band Elimination Filter Employing Surface Acoustic Wave Resonator" Electronics Letters 1975, Vol 11, pp. 108-109, discusses the use of SAW impedance elements to facilitate notch filters in the UHF range.
Two problems generally exist with the prior art notch filters. The first is that the insertion loss of the filter is typically fairly high and the second is that the bandwidth of the notch is typically very narrow because the transmission phase characteristics of the surface acoustic wave are changing rapidly with frequency and hence the bridge is only in balance over a very narrow region where the transmission phase of the surface acoustic wave is exactly correct.
The first problem, insertion loss, has been previously overcome by using a low loss surface acoustic wave device such as a surface acoustic wave resonator. Unfortunately, use of a surface acoustic wave resonator makes the second problem, bandwidth, somewhat worse because the phase slope in the transmission response is very steep in a surface acoustic wave resonator structure. In principle, one could use a low-loss surface acoustic wave filter and thereby have a somewhat less steep phase slope but any two-port surface acoustic wave device will always have a finite phase slope simply because the phase slope represents the time delay between the input and output transducer. The input and the output transducers must always be physically separated from one another on the surface acoustic wave device surface. Thus, it is impossible to achieve a two-port surface acoustic wave device in which both the transmission magnitude and the transmission phase are constant over a reasonable bandwidth.
In many cases, it is desired to have a notch filter where the bottom of the notch has a finite width to allow for temperature drift or other instability in either the notch filter or the signal to be rejected by the notch. The present invention utilizes a SAW device whose input admittance is relatively constant in both magnitude and phase across the bandwidth of the notch thus resulting in a notch filter with broader rejection characteristics than are typically achieved in the prior art. When this type of device is used in a bridge configuration the "balance condition" which results in the notch is then achieved over a broader bandwidth.